The invention relates to a device for reducing the amplitude range of three colour signals representing an image, the three signals being supplied by any source, such as a camera, a television film scanner, etc. These signals may have an amplitude range higher than that acceptable by presently used recording or transmission means. In order to restore a correct image, it is necessary to reduce the amplitude range of these signals prior to transmission or recording, so that their values never exceed a fixed maximum value. Since the very high amplitude values are statistically not very numerous, they are sacrificed to the benefit of medium and low values, making it possible to reconstitute the essential information in the image. However, in order not to loose all the information corresponding to the peaks of these high amplitude signals, the high amplitude are compressed rather than clipped.
In order to carry out a reduction in the dynamic range by compressing the highest values, it is known to use three identical, non-linear compression circuits, which respectively process the three colour signals. Their transfer function has a graph constituted by two straight line segments. The first segment, corresponding to the low and medium signal values, has a unitary slope, while the second segment, corresponding to the high signal values, has a slope which is below unity. It is possible to regulate the value at which the compression commences and the value of the slope of the second segment.
When the three colour signals represent achromatic image elements, the three colour signals have the same value. Therefore the signals restored by the compression circuits have the same value and the elements of the reconstituted image remain achromatic. However, colour signals corresponding to coloured elements do not have the same value and their value differences are reduced when one or more of the signal amplitudes is sufficiently high for a compression circuit to perform a compression. The values of the three signals then tend to become equal at the output of the compression circuits and the restored image elements thus tend to become achromatic. The phenomenon becomes even more visible as the colours become less saturated, i.e. flesh tint, blue sky, pastel shade, etc.